Staphylococcus aureus Esx Factors Control Human Dendritic Cell Functions Conditioning Th1/Th17 Response

Abstract

The opportunistic pathogen Staphylococcus aureus (S. aureus) is a major cause of nosocomial- and community-acquired infections. In addition, many antibiotic-resistant strains are emerging worldwide, thus, there is an urgent unmet need to pinpoint novel therapeutic and prophylactic strategies. In the present study, we characterized the impact of infection with the pandemic methicillin-resistant USA300 S. aureus strain on human primary dendritic cells (DC), key initiators and regulators of immune responses. In particular, among staphylococcal virulence factors, the function of EsxA and EsxB, two small acidic dimeric proteins secreted by the type VII-like secretion system Ess (ESAT-6-like secretion system), was investigated in human DC setting. A comparative analysis of bacterial entry, replication rate as well as DC maturation, apoptosis, signaling pathway activation and cytokine production was performed by using wild type (wt) USA300 and three isogenic mutants carrying the deletion of esxA (ΔesxA), esxB (ΔesxB), or both genes (ΔesxAB). The S. aureus mutant lacking only the EsxA protein (ΔesxA) stimulated a stronger pro-apoptotic phenotype in infected DC as compared to wt USA300, ΔesxAB, and ΔesxB strains. When the mutant carrying the esxB deletion (ΔesxB) was analyzed, a higher production of both regulatory and pro-inflammatory mediators was found in the infected DC with respect to those challenged with the wt counterpart and the other esx mutants. In accordance with these data, supernatant derived from ΔesxB-infected DC promoted a stronger release of both IFN-γ and IL-17 from CD4+ T cells as compared with those conditioned with supernatants derived from wild type USA300-, ΔesxAB-, and ΔesxA-infected cultures. Although, the interaction of S. aureus with human DC is not yet fully understood, our data suggest that both cytokine production and apoptotic process are modulated by Esx factors, thus indicating a possible role of these proteins in the modulation of DC-mediated immunity to S. aureus.

Dendritic Cells Internalize Staphylococcus aureus More Efficiently than Staphylococcus epidermidis, but Do Not Differ in Induction of Antigen-Specific T Cell Proliferation

“…Although the capacity of DCs to kill the internalized staphylococci was limited, we did not observe any intracellular net growth of the bacteria. In line with our findings, previous studies demonstrated that human DCs have a low efficiency of killing internalized pathogens, especially when compared to human monocytes and macrophages [73][74][75]. This result is in agreement with the main function of DCs, which is to sense and process pathogens and present their antigens to T cells, rather than to eliminate all pathogens.…”

Flotillin scaffold activity contributes to type VII secretion system assembly in Staphylococcus aureus

“…Targeting of flotillin scaffold activity could be an appropriate strategy for fighting bacterial infection by perturbing oligomerization of multiple virulence-related protein complexes such as T7SS. In mice, the T7SS Esx substrates participate in formation of persistent abscesses, probably due to the virulence of EsxA, EsxB, EsxC and EsxD secreted proteins [ 31 , 32 , 37 , 38 , 66 ]. In addition, EsxC is an immunogenic substrate, and kidney abscesses are associated with the generation of anti-EsxC antibodies [ 32 ].…”

The type VII secretion system protects Staphylococcus aureus against antimicrobial host fatty acids

“…Indeed, S. aureus mutants lacking the entire T7SS 7 or specific T7SS components (EsxA, EssB, EssC, EsxC, EsxB, EsaB, EsaD or EsaE) were consistently shown to be less virulent and/or persistent in various mouse infection models 11 , 17 – 21 . EsxA is necessary to delay apoptosis of S. aureus -infected epithelial and dendritic cells, while other substrates modulate cytokine production 18 , 22 , 23 . Although the relevance of T7SS to S. aureus is less understood, a role for the toxin-antitoxin pair EsaD (or EssD) and EsaG (or EssI) was recently demonstrated in intraspecies competition 9 , 15 .…”